This investigation will differentiate between psychological stress and physical strain (or exertion), and test their contributions to preterm delivery and low birth weight. Both factors are associated with poor perinatal outcome. However, few studies to date have focused on separating their confounding effects on low birth weight and preterm delivery while examining those of other determinants (e.g. medical complications, demographics, or health behaviors). A conceptual model has been developed based on the existing empirical literature and the previous research of the principal investigators, Hobel and Dunkel-Schetter, in which the effects of psychological stress (conceptualized as perceived chronic stress), and the effects of physical exertion (conceptualized here as physical strain in activities of daily living) on time of delivery and birthweight will be examined. In addition, biochemical, biophysical, and behavioral mechanisms hypothesized to mediate the effects of these variables on outcomes are to be tested. These issues will be addressed in a prospective and longitudinal study of 500 women, 18 years or older, recruited from public, private, and managed health-care settings. The sample will be diverse with respect to ethnicity, maternal risk conditions, and parity. Women will be entered into the study up to 20 weeks gestation. Data will be collected at time of entry into the study, on two successive occasions prior to delivery, and once postpartum by interview to assess the two major study variables (perceived chronic stress and reported physical strain). Biochemical mediators will be measured by analysis of urinary catecholamines and cortisol. Biophysical parameters to be obtained are fetal growth, fetal state, and uterine activity. In addition, substance use and social support will be assessed with interviews on these occasions to test whether they mediate the hypothesized relationships between predictors and outcomes. Outcomes will be obtained from the labor, delivery, postpartum and newborn data systems.